1. Field of the Invention
The present invention relates generally to a dynamic seal, and more specifically to card seal for use in turbo machines such as gas turbines, steam turbines, compressors, pumps, etc.
2. Description of the Related Art including information disclosed under 37 CFR 1.97 and 1.98
A gas turbine engine includes a compressor and a turbine each having multiple rows of rotor blades interspaced between stator or guide vanes. In-between each row or stage, a large pressure differential is formed. In the compressor, the pressure increases in the flow direction while in the turbine the pressure decreases. The pressure differential between adjacent stages in the compressor is smaller than in the turbine because of the greater number of stages used in the compressor.
A dynamic seal is used between the rotor and the stator of the turbomachine to limit leakage in order to improve the efficiency of the turbo machine. In the turbine, the leakage is from the hot gas flow passing through the turbine. Hot gas leaking into the rim cavity will expose the rotor disks to the extreme hot temperature. Thus, better seals reduce leakage to increase performance of the turbo machine and to prevent over-exposure of turbomachine parts to excessive temperatures. In one example, the rotor disks in the turbine are made from a high strength material different than the rotor blades or stator vanes that would develop cracks due to thermal stress loads if exposed to high temperature from excessive hot gas leakage into the adjacent rim cavity. This is why purge air is often used in the rim cavities to push out or dilute and hot gas flow leakage that leaks through the dynamic seal and into the rim cavity.
In a turbine of a gas turbine engine, labyrinth seals or brush seals are used for the dynamic seals. In some cases, a combination of brush and labyrinth seals is used because of the characteristics of each. A labyrinth seal makes a good seal at relatively high rotational speeds while the brush seal is best for relatively low rotational speeds. This is due to the use of brushes that rub against the rotating part formed by the dynamic seal. At higher rotational speeds, the brushes will wear out early. Brush seals have less leakage than labyrinth seals, but wear out easily when rubbing at higher speeds. One reason why a turbine uses combinations of labyrinth and brush seals is due to engine transients, which is when the engine is stopped and then restarted. The rotor shaft and the engine casing are made of different materials that have different coefficients of thermal expansion. Thus, the parts grow in a radial direction at different rates due to heat transfer to or from the part. Labyrinth seals are also capable of sealing much higher pressure differentials than brush seals.
A card seal is formed of a number of flat leafs or cards arranged around a rotor shaft in an annular formation in which a gap formed between adjacent cards due to surface irregularities and is generally parallel to the rotor shaft axis. Each plate is capable of sliding over adjacent cards so as to maintain contact with the rotor shaft surface or float on top thereof. An outer end of the cards is held in a casing while the inner ends float or make contact with the outer surface of the rotor shaft. One side of the cards is exposed to the high pressure side while the other side is exposed to the low pressure. U.S. Pat. No. 6,736,597 issued to Uehara et al on May 18, 2004 and entitled AXIS SEAL MECHANISM AND TURBINE shows one such card seal. This card seal will allow for too much leakage through the small gaps formed between adjacent cards to be useful in the gas turbine engine.
Leaf or card seals have been developed in order to provide a better seal that includes benefits from both the lab seals and the brush seals. Card seals are primarily utilized to maintain a pressure barrier between two cavities created by a static structure, a moving structure and separated by the seal structure. Specifically, a static cylindrical case, a rotor and the seal are some examples. Whereas a solid seal structure—such as a ring seal—would undergo severe contact loads due to rotor lateral excursions, card seals are designed to be compliant and either tolerates small contact or bend out of the rotor path.
The prior art card seal structures includes a plurality of thin cards arranged so that the weak axis of bending is presented in the direction of rotor motion. A tilt or lean in direction of motion is included so that contact occurs at an angle to the direction of motion. The net effect of these two features is to minimize wear by reduced contact load and actualize displacement due to boundary layer air pressure which will provide an air cushion between moving surface and free edge of the cards if a flexible card is used.
Therefore, by design the plurality of cards that form the card seal provides for a direct leakage path between the cards. For this reason, the cards are reinforced along the plate longitudinal axis by either changing the plate number density or forming a localized deformation such as dimpling or ridges on each thin plate that align with each other. Prior art card seals include a plate with a raised middle portion that extends along the longitudinal axis of the plate that will reinforce the plate and provide a barrier to the direct leakage flow path between the flat cards. The leakage area in this card seal is reduced by the cross sectional area of the ridge. However, the ridge or stamped process may effectively increase the weak axis inertia and impede the plate's ability to bend away from the rotor lateral excursions. In other words, the cards having the ridges for added strength will produce a more rigid (or, less flexible) card seal.
Since card seals are formed from an annular arrangement of loose fitting cards or leafs, assembling the card seal can be difficult in that individual cards can fall out of place during the assembly process. Also, during operation of the card seal, especially for a card seal used in a relatively high temperature environment such as a gas turbine engine, the case that holds the card seal could grow thermally much more than the rotor shaft in which the card seal forms the seal. Thus, the radial distance between the card seal case and the card seal cards inner ends could expand enough to significantly reduce the sealing effect of the card seal.